When I was in high school, my friend and I played a robot fighting game, where you had some fixed energy and every action would consume some of it. My friend fo

When I was in high school, my friend and I played a robot fighting game, where you had some fixed energy and every action would consume some of it. My friend found a way to exercise a large number of expensive actions in one turn, resulting in energy underflow. With some care, he found a way to achieve the maximum possible energy. At no point was the energy out of bounds. Sanity checks can detect invalid states, but not invalid calculations (which have intermediate values that may exceed the bounds - consider calculating the average stat in your Dark Souls example). You can get an unexpectedly valid state from a calculation that overflows, and thereby pass a manual safety check. Trapping arithmetic will catch these.